1. Field of the Invention
The present invention relates to concurrent and simultaneous steam generation and recovery of vanadium and nickel from fluid coke or flyash derived from the burning of fluid coke.
2. Information Disclosure Statement
In the conventional method of vanadium recovery and steam generation as described in Whigham Canadian Pat. No. 819,099 and in anonymous report entitled "Fina Launches Petrochemical Vanadium", in Canadium Petroleum, page 67, September 1966, fluid coke derived from thermal cracking of crude oil is first combusted with air in a steam boiler to generate process steam. Secondly, the hot and fine flyash must be collected in electrostatic precipitators. Thirdly, the collected flyash is slurried with sulfuric acid solution and subjected to a low pressure leach. The leached vanadium oxide is then oxidized to pentoxide. Finally, the pentoxide is precipitated by neutralizing part of the sulphuric acid.
This conventional process is found to be ineffective for vanadium recovery from fluid coke which is derived from oil sand bitumen. Recovery of vanadium is about 30-40 percent. This poor performance of vanadium recovery with the conventional method was well documented by L. A. Walker et al in "Potential for Recovering Vanadium from Athabasca Tar Sand", 26th Canadian Chemical Engineering Conference, Toronto, Oct. 3-6, 1976. This poor performance is attributed to vitrification or encapsulation of the vanadium compound in glassy alumina silicate material during the high temperature, ie, .gtoreq.1600.degree. F. combustion step. The alumina silicate compound is absent in conventional crude oil, but is present in the bitumen derived from oil sands. As described by Stemerowicz et al, "Recovery of Vanadium and Nickel from Athabasca Tar Sand Fly Ash", CIM Bulletin, pages 102-108, April 1976, micrographs of the resultant flyash invariably show glassy beads of non-porous material which effectively slow the infiltration and hence leaching action of sulphuric acid.
Three new processes have been suggested recently to circumvent the vitrification problems:
(1) as described by T. R. Jack et al, "Leaching of Vanadium and Other Metals from Athabasca Oil Sands Coke and Coke Ash", Fuel, 58, page 589, 1979, the coke can be leached directly with sulphuric acid; PA1 (2) according to P. J. Griffin et al, "Extraction of Vanadium from Oil Sands Fly Ash", in "Waste Treatment and Utilization, Theory and Practice of Waste Management", Volume 2, Moo-Young et al, editors, Pergamon Press (1981) the collected fly ash can be roasted at 800.degree.-900.degree. C. (1470.degree.-1660.degree. F.) with a sodium salt, and the resultant molten salt is then cooled and subjected to NaOH leaching; the same authors also presented the concept in a paper entitled "Extraction of Vanadium and Nickel from Athabasca Oil Sands Fly Ash" at the Second International Conference on Heavy Crude and Tar Sands (UNITAR) 1981; and PA1 (3) the carbon in the flyash is separated by flotation and the ash is then smelted to form alloys of vanadium, nickel, iron and carbon, according to the Stemerowicz et al reference cited above. PA1 a. Fluid coke may contain sulphur at concentrations up to about 10 percent by weight. If the coke is burned in a conventional boiler, sulphur dioxide from the combination of sulphur and oxygen will be emitted with the boiler off-gas. Sulphur dioxide is a gaseous pollutant which must be removed to comply with pollution control regulations. PA1 b. The flyash liberated during the combustion step is a fine powder (&lt;325 mesh) at high temperature (.gtoreq.1600.degree. F.) Expensive equipment demanding high energy input, ie. electrostatic precipitators, are needed for 98% plus recovery. Uncollected flyash is also a recognized particulate pollutant if emitted to the atmosphere; such emissions are subject to regulatory controls. PA1 c. Combustion of fluid coke in a steam boiler is generally inefficient in terms of carbon utilization; about 60% of the input carbonaceous material remains unburned in the flyash. If this carbonaceous material is permitted to go through the subsequent leaching step, it would unnecessarily take up process space. Furthermore, it would be lost with the unleachable ash waste, ie., loss of 60% of the available thermal energy contained in the coke. PA1 1. Direct leach of fluid coke. Since only a minute quantity of vanadium is present in the fluid coke, a large amount of material has to be processed in large process vessels, resulting in high capital investment and low extraction efficiency. The rate of extraction is further decreased due to the low specific surface area of fluid coke. Following acid leaching, the carbonaceous material will be wet and must be dried before it can be combusted in steam boilers. This, of course, results in lower net steam output per unit of fluid coke, compared with the conventional process. PA1 2. Roasting with sodium salt. This is an energy intensive process; a large amount of energy must be put into a process vessel in order to achieve fusion of flyash and the sodium salt. The fused material will also be low in porosity. Generally, the total specific surface area of non-porous materials is much lower than that of porous material. The rate of leaching is normally governed by the surface area available to the leaching agent. A lower specific surface area will result in a slower rate of metal extraction. As a result, residence time has to be increased, resulting in larger process vessels for the same percentage of recovery. Furthermore, the leaching solution (sodium hydroxide) is non-indigenous to the process and must be supplied from outside, necessitating storage and transportation systems. PA1 3. Smelting to form ferro-alloys. This is also an energy-intensive process. Furthermore, it suffers from low percentage of recovery, ie, &lt;60%, and the alloy product requires extensive further processing and refining to product marketable products, ie., ferrovanadium and ferronickel alloys. PA1 (a) slurrying the feed material, ie, fluid coke or fly ash in water and subjecting the slurried material to elevated temperature and pressure in the presence of oxygen in a wet oxidation reactor to generate, in situ, a liquor containing sulphuric acid and soluble compounds of vanadium and nickel, and simultaneously produce heat for co-generation of steam; PA1 (b) removing suspended solids from the liquor; PA1 (c) sequentially precipitating, removing and recovering vanadium and nickel products in concentrated form from the liquor; and PA1 (d) recovering energy from the reactor off-gases by heat exchange to produce steam, thereby cooling the off-gases.
The conventional process of vanadium recovery from flyash derived from regular crude oil suffers the following three disadvantages:
The three recently proposed processes discussed above also have inherent disadvantages, as listed below:
The present invention is an integral and simultaneous steam generation and metal (vanadium and nickel) recovery process, which eliminates or minimizes gaseous and particulate pollutants. One step of the process consists of wet oxidation, a method commonly used for disposal of sewage sludges, paper mill sludges and chemical/petrochemical wastes. Wet oxidation of such materials is shown for example in Zimmermann U.S. Pat. No. 2,824,058.
Production of sulphuric acid from elemental sulphur by wet oxidation is shown by Schoeffel U.S. Pat. No. 3,042,489.
In Schotte U.S. Pat. No. 3,649,534, the dewaterability of waste activated sludge is shown to be enhanced by wet oxidation of the sludge following acidification to pH 2-5.
Fassell et al U.S. Pat. No. 3,870,631 discloses the acidification of combustible organic matter, in particular sludge, sewage or the like, to pH 1.5-7.0 for the purpose of accelerating the combustion rate in a wet oxidation system.
When applied to ore processing, the wet oxidation process comprises pressure oxidation leaching under conditions such that comsiderable oxidation of oxidizable matter takes place. McGauley U.S. Pat. No. 2,588,265 shows an oxidative pressure leaching of nickel-bearing ore at 275.degree.-750.degree. F. and at a pH less than about 3.0.